On Thu, 30 May 2013, Rafael Aquini wrote:
> read_swap_cache_async() can race against get_swap_page(), and stumble across
> a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> swapcache yet. This transient swap_map state is expected to be transitory,
> but the actual placement of discard at scan_swap_map() inserts a wait for
> I/O completion thus making the thread at read_swap_cache_async() to loop
> around its -EEXIST case, while the other end at get_swap_page()
> is scheduled away at scan_swap_map(). This can leave the system deadlocked
> if the I/O completion happens to be waiting on the CPU workqueue where
> read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
>
> This patch introduces a cond_resched() call to make the aforementioned
> read_swap_cache_async() busy loop condition to bail out when necessary,
> thus avoiding the subtle race window.
Yes, I never realized this at the time I inserted discard there.
As you know, Shaohua has a better swap discard implementation, which
avoids the problem by using SWAP_MAP_BAD, but this cond_resched() is
a good simple workaround for now - thanks.
>
> Signed-off-by: Rafael Aquini <aqu...@redhat.com>
Acked-by: Hugh Dickins <hu...@google.com>
Cc: sta...@vger.kernel.org
> ---
> mm/swap_state.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index b3d40dc..9ad9e3b 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry,
> gfp_t gfp_mask,
> * Swap entry may have been freed since our caller observed it.
> */
> err = swapcache_prepare(entry);
> - if (err == -EEXIST) { /* seems racy */
> + if (err == -EEXIST) {
> radix_tree_preload_end();
> + /*
> + * We might race against get_swap_page() and stumble
> + * across a SWAP_HAS_CACHE swap_map entry whose page
> + * has not been brought into the swapcache yet, while
> + * the other end is scheduled away waiting on discard
> + * I/O completion.
> + * In order to avoid turning this transitory state
> + * into a permanent loop around this -EEXIST case,
> + * lets just conditionally invoke the scheduler,
> + * if there are some more important tasks to run.
> + */
> + cond_resched();
> continue;
> }
> if (err) { /* swp entry is obsolete ? */
> --
> 1.8.1.4
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